Many computer printers, including some low resolution ink-jet printers, scan a print head back and forth relative to a print medium to print graphics and text images thereon. Printing typically occurs while the print head is scanned in each direction, thereby employing relatively fast bidirectional printing.
An ink-jet printer ejects ink drops from the print head onto the print medium to form a printed image. The print head is typically spaced apart from the print medium, and the droplets are ejected toward the print medium at a relatively low velocity. Accordingly, there is a propagation time during which the droplets propagate from the print head to the print medium. The propagation time is dependent upon the velocity at which the droplets are ejected from the print head and the distance between the print head and the print medium.
The print head and print medium move relative to each other at a scanning velocity. A droplet projected from the moving print head will have the scanning velocity in the direction the print head is being moved. A droplet projected toward an image location on the print medium must, therefore, be ejected from the print head at an ejection time that occurs before the print head is aligned with the image location. Nominally, the ejection time precedes the alignment of the print head with the image location by about the propagation time of the droplet.
When printing takes place in only one scan direction, all droplets are subjected to the same scanning velocity. As a result, the alignment of droplets ejected during successive scans is substantially independent of the propagation time of the droplets.
In bidirectional printing, however, droplets are subjected to different scanning velocities during the successive scans in opposite directions. As a result, the alignment of droplets ejected during successive scans is dependent upon the propagation time of the droplets (i.e., the velocity at which the droplets are ejected from the print head and the distance between the print head and the print medium). Therefore, unidirectional printing provides potentially greater printing quality, albeit at a loss of printing speed.
The droplet ejection velocity can be regulated by the print head. Accordingly, the distance between the ink-jet print head and the print medium must be accurately maintained to provide adequate alignment of the droplets ejected during successive scans in opposite directions.
High-resolution ink-jet printers can form images with ink drops spaced apart by about 120 dots per centimeter. Maintaining such resolution requires that the distance between the print head and print medium be maintained within a tolerance of about .+-.0.05 mm. However, such printers are sometimes adapted to print onto media having a wide range of thicknesses, creating a drop alignment problem for bidirectional printing.
High-resolution ink-jet printers also require high positioning accuracy and repeatability of the print head relative to the print medium. There are many prior apparatus and methods for positioning a print head relative to a print medium. For example, in a model 4692 ink-jet printer, manufactured by the assignee of this application, a print medium is clamped to a rotating drum and the print head is moved one incremental position parallel to the axis of drum rotation for each rotation of the drum. The print head includes four nozzles aligned in the direction of drum rotation for printing color images having a resolution of 60 dots per centimeter. The print head is positioned by a stepper motor coupled to the print head by a cogged belt and requires about four minutes to print an image.
Referring to FIG. 1, a reciprocating print head positioning example is described in U.S. Pat. No. 5,227,809 issued Jul. 13, 1993 for AUTOMATIC PRINT HEAD SPACING MECHANISM FOR Ink-jet PRINTER, assigned to the assignee of this application, in which an ink-jet printer 10 requires two minutes to print a 120 dot per centimeter color image. An ink-jet print head assembly 12 supports a print head 14 having 96 orifices from which ink droplets are ejected toward a print medium 16 that is mounted on a drum 20. Print medium 16 is fed through a pair of media feed rollers 22a and 22b and secured to drum 20 by a media securing system 24. Securing system 24 includes a media clamp 26 that receives and clamps a leading end of print medium 16 against drum 20. Media clamp 26 slides into and remains stationary within a slot 28 in drum 20.
A drum motor (not shown) incrementally rotates drum 20 in a direction 34 about an axis 36 of drum 20, thereby pulling print medium 16 through media feed rollers 22a and 22b and under a back tension blade 38 that is spring biased toward drum 20. Print medium 16 slides under and is held against drum 20 by back tension blade 38 as drum 20 rotates.
A print head positioning system 50 includes a carriage 52 slidably mounted on a pair of guide rails 54a and 54b and supporting print head assembly 12. A carriage drive belt 56 is attached to carriage 52 and held under tension by a pair of belt pulleys 58a and 58b. A carriage stepper motor 60 linked to pulley 58a drives carriage 52 in directions 62a and 62b along guide rails 54a and 54b. When printing images on print medium 16, the drum motor incrementally rotates drum 20 about axis 36 while carriage motor 60 bidirectionally drives carriage 52 along guide rails 54a a and 54b and a printer controller 70 delivers print control signals to a control input 72 of print head 14 which ejects ink droplets toward print medium 16. The print control signals are delivered to print head 14 while carriage 52 is driven in both directions 62a and 62b, thereby providing bidirectional printing in which successive bands of image lines are printed alternately in directions 62a and 62 b by the multiple nozzles of print head 14.
Printer 10 suffers from a number of disadvantages including a complex print medium handling mechanism, susceptibility to bidirectional dot misconvergence, and a relatively slow printing speed.
Printing speed can be increased by increasing the number of nozzles in print head 14, but even with 124 nozzles, printer 10 still requires one minute to print an image.
Printing speed can also be increased by increasing the velocity at which carriage 52 reciprocates back-and-forth in directions 62a and 62b. However, drop convergence problems increase with carriage speed, and positioning accuracy decreases because of dynamic positioning problems associated with rapidly moving the relatively massive ink-jet print head assembly 12.
Prior work directed to improving the speed and accuracy of reciprocating print head movement is described in U.S. Pat. No. 4,939,440 issued Jul. 3, 1990 for FRICTION-COMPENSATING MASS MOTION CONTROLLER, U.S. Pat. No. 4,957,014 issued Sep. 18, 1990 for CABLE DRIVE GEOMETRY, and U.S. Pat. No. 5,036,266 issued Jul. 30, 1991 for MASS VELOCITY CONTROLLER, all of which are assigned to the assignee of this application. Unfortunately, all the reciprocating print head positioning techniques are eletro-mechanically complex, costly, and do not really solve the printing speed, bidirectional convergence, or paper path complexity problems.
For the above-described reasons, a transfer printing process similar to one described in U.S. Pat. No. 4,538,156 issued Aug. 27, 1985 for Ink-jet PRINTER is desirable for increasing printing speed, eliminating bidirectional convergence problems, and reducing paper path complexity. A transfer printer employs a print media-width print head that ejects image forming droplets directly onto a rotating drum. After the drum is "printed," a print medium is placed in rolling contact with the drum such that the image is transferred from the drum to the print medium.
FIG. 2 shows that the transfer printer includes a transfer drum 80 rotated by a motor 82 in a direction indicated by an arrow 84. A print head assembly 86 includes a frame 88, guide bars 90 and 92, a nozzle array 94, a stepper motor 96, a belt 98, and a lateral positioning assembly 100. An ink reservoir 102 is connected to nozzle array 94 by a tube 104.
The transfer printer also includes a print media supply surface 106, a printing pressure roller 108, and a drum cleaning assembly 110. A drum cleaning web 112 and transfer drum 80 are brought into contact by a roller 114 that is moved toward transfer drum 80 in proper time relationship with movement of printing pressure roller 108. Cleaning web 112 prepares the surface of transfer drum 80 to receive the ink drops from nozzle array 94.
Nozzle array 94 is a print media-width linear array of nozzles spaced apart by 0.254 millimeter to print a 79 dot per centimeter resolution image on drum 80 during 20 successive rotations of transfer drum 80. The image on transfer drum 80 is transferred when a print medium 115 is advanced into a nip formed between printing pressure roller 108 and transfer drum 80.
Transfer drum 80, print head assembly 86, and drum cleaning assembly 110 are mounted between two frame plates of which only a right-hand plate 116 is shown.
FIG. 3 shows lateral positioning assembly 100 in greater detail. Stepper motor 96 incrementally moves print head assembly 86 to access successive printing tracks on transfer drum 80. Thereby, nozzle array 94 is moved laterally on guide rods 90 and 92 under the influence of the lateral motion assembly 100. The rotation of stepper motor 96 is transferred to a shaft 120 by belt 98 and a pulley 122. Threads 124 on shaft 120 engage internal threads 126 on a nut 128. Nut 128 and a body 130 are held in a fixed relationship by splines (not shown) and by a spring 132.
The printing tracks on transfer drum 80 are successively accessed by energizing stepper motor 96 for a predetermined number of steps sufficient to achieve the desired lateral motion of the print head assembly 86. After each nozzle of nozzle array 94 has printed all tracks of a corresponding succession of tracks, stepper motor 96 is reversed to cause body 130 and print head assembly 86 to return to an initial printing position. A return spring 134 cooperates with spring 132 to ensure accurate positioning of nozzle array 94 by eliminating play in the meshing of threads 124 on shaft 120 with internal threads 126 on nut 128. Body 130 of lateral motion assembly 100 is moved laterally on guide rods 136 and 138. Lateral movement of body 130 is coupled by a pin 140 to a tab 142 that is attached to print head assembly 86.
The above-described transfer printer is advantageous because of rapid unidirectional printing, constant print head to media spacing, insensitivity to print media thickness, and a greatly simplified "straight through" paper path.
However, lateral motion assembly 100 is relatively complex, expensive, and unable to uniformly, accurately, and repeatably position a print head assembly with a nozzle array capable of printing 118 dot per centimeter images. This is because anti-backlash springs 132 and 134, belt 98, pulley 122, threads 124 and 126, and guide rods 90, 92, 136 and 138 cause an unacceptable degree of friction and dimensional tolerance buildup that causes "print banding" in high-resolution images. Moreover, the stepper motor- and lead-screw-based print head positioner is not readily adjustable to compensate for the friction and tolerance buildups.
What is needed, therefore, is a print head assembly positioner that is simple, adjustable, free of most friction and backlash, and capable of reliably supporting high-resolution printing applications without visible print banding or other print artifacts.